Functional MRI and Structural Connectome Analysis of Language Networks in Japanese-English Bilinguals

Abstract

We aimed to clarify the mechanisms of neural plasticity involved in language. We hypothesized that alterations which occur in bilinguals could reflect the mechanisms of acquisition of a second language and simulate neural plasticity related to language. We compared spatial characteristics of story listening-related hemodynamic modulations and subcortical fiber networks between monolinguals and bilinguals. Participants were Japanese monolinguals and Japanese-English bilinguals whose first language was Japanese. We divided bilinguals into early and late bilinguals depending on whether the age of acquisition was before after 7 years of age. We applied intergroup analysis to investigate the following: (1) blood oxygen level-dependent response (BOLD) responses during story listening by block-based fMRI; (2) number of fibers (NOFs) between specific edges by DTI. Both bilingual samples showed larger BOLD responses (BRs) in the right putamen and bilateral superior temporal gyri compared to the Japanese monolinguals in fMRI. Late bilinguals demonstrated bigger BRs in the right anterior temporal lobe and left medial parietal lobe than early bilinguals. Early bilinguals showed a higher NOFs between the right putamen and precentral gyrus than monolinguals and late bilinguals in DTI. Late bilinguals showed a lower NOFs between the left superior temporal gyrus and supramarginal gyrus than monolinguals and early bilinguals. Early bilinguals reinforce the subcortical fiber network between the right putamen and precentral gyrus, and activate the right putamen to gain alternative language function. We conclude that these key cerebral regions and subcortical fiber networks could contribute to the neural plasticity of language.

Keywords: bilingualism; connectome; default mode network; functional MRI; neural efficiency hypothesis; neural plasticity.